1. Field of the Invention
The present invention relates to metal-organic complex oxidation catalysts and, more particularly, to a combination of a dinuclear transition metal-tetra-diisocyanide complex with a stonger oxidant.
2. Description of the Prior Art
Dinuclear transition metals complexed with four binucleating diisocyanides bridge ligands have previously been reported. The dirhodium tetra-diisocyanopropane dimers undergo two-center oxidative addition reactions with several substrates. The orbital interactions between the directly coupled metal centers give rise to striking electronic absorption properties, the most prominent being a low-lying system attributable to the .sup.1 A.sub.1g .fwdarw..sup.1 A.sub.2u (1a.sub.2u .fwdarw.2a.sub.1g) excitation.
The dimeric complex are very interesting oxidizing agents in multielectron redox processes since each metal center can furnish or remove one or more electrons from a substrate. Though analogous monomeric metal complexes are not good oxidation catalysts, the dimers have shown good oxidation properties with numerous substrates, probably due to the capability of the binucleating ligands to maintain or reduce metal-metal spacing on oxidation. However, the redox reactions are stoichiometric requiring reoxidation of the complex. The complex has been shown to be readily oxidized by halogens such as bromine, chlorine or iodine. Oxidation by molecular oxygen or air would be desirable for industrial processes due to formation of a peroxide intermediate at low cost. However, the direct oxidation by oxygen is relatively slow due to the large energy barrier for this reaction. Peroxidic intermediates are also of value due to the selectivity in certain reactions such as in the oxidation of olefins such as propylene to propylene oxide under mild conditions while minimizing formation of by-products.
Present commercial processes for producing propylene oxide suffer from one or more major drawbacks. The chlorohydrin process produces chlorine compounds that pose pollution problems. The oxirane process produces about twice as much styrene or t-butanol co-product as propylene oxide. Other processes under consideration such as peracid or hydrogen peroxide also produce co-product and/or require hydrogen peroxide, an expensive reagent.